1. Field of the Invention
The present invention relates to a thermal printing device for printing an image data on a thermosensible paper.
2. Description of the Related Art
The inventor of the present invention knows a facsimile apparatus now widely used for sending or receiving image data such as characters or graphics to or from remote devices through a public phone circuit. Such the facsimile apparatus includes a thermal printing device which prints a converted image data on a thermosensible paper. The converted image data is converted by a facsimile receiver which receives electric picture signals and performs a conversion of the received electric picture signals to the image data. The thermal printing device has a platen roller and a thermal head between which a thermosensible recording paper is tightly held. The converted image data is printed on the thermosensible paper tightly held therebetween. The thermosensible paper is rolled and accommodated with in the facsimile apparatus.
In order to reliably print the converted image data received intermittently, the rolled thermosensible paper is stocked sufficiently. Further, it is necessary to prevent printing disability caused by exhaust of ink and operating failure caused when the facsimile apparatus itself is moved, for example.
In case that the image data is printed when the thermosensible paper is hardly left, the thermosensible paper may be used up while the image data is being printed. So, in case that the thermosensible paper is hardly left, the thermal printing device takes the steps of sensing a mark indicating that the remaining length of the thermosensible paper is less than a predetermined length, displaying a message that the thermosensible paper should be replaced, and stopping printing of the image data until the thermosensible paper is newly replaced.
That is, when the remaining length of the thermosensible paper is less than the predetermined length, as mentioned above, the thermal printing device keeps a waiting state until the thermosensible paper is replaced. Printing on the thermosensible paper is carried out in the state where the thermosensible paper is tightly held between the platen roller and the thermal head so that the thermal head is pressed against the platen roller. When waiting for the new thermosensible paper to be replaced, the thermal head is being pressed against the platen roller. Hence, when the thermosensible paper is replaced, it is necessary to take troublesome steps of taking out the old thermosensible paper, mounting the new thermosensible paper, and pressing the thermal head against the platen rollers with the thermosensible paper laid therebetween again.
Since the facsimile apparatus intermittently receives the electric picture signals, no one can estimate when a replacement of the thermosensible paper is required. Therefore, some unflavorable situations may take place such as where the replacement of the thermosensible paper is required while the electric picture signals are being received and where a user unfamiliar to handling of the thermosensible paper has to replace the thermosensible paper. In such situations, the burdensome operation is that the thermosensible paper has to be released out of the pressure given by the thermal head and the platen roller. If this operation is skipped, the portion of the thermosensible paper tightly held between the thermal head and the platen roller is torn out when the old thermosensible paper is being replaced, resulting in disadvantageously needing more troublesome operation for removing the torn paper.
In turn, a second disadvantage of the thermal printing device will be described.
As mentioned above, the thermal printing device included in the facsimile apparatus, for example, serves to tightly hold the thermosensible paper between the platen roller and the thermal head. When the image data is received, the platen roller is rotated so that the thermosensible paper is conveyed in one direction and heater elements included in the thermal head is actuated by an electric power. The heater elements are arranged in a vertical manner to the conveying direction of the thermosensible paper and are selectively heated in synchronous to the paper conveyance, resulting in printing the image data on the thermosensible paper.
The thermal printing device of the facsimile apparatus provides a mechanism for temporarily reducing a pressure constantly kept between the thermal head and the platen roller or moving the thermal head for releasing the pressure kept therebetween. For example, the thermosensible paper is formed so that it can fit in a cabinet of the facsimile apparatus. By opening a part of the cabinet, the pressure applied against the thermal head is released. By closing the cabinet, the pressure is applied to the thermal head so that the thermosensible paper is tightly held between the platen roller and the thermal head. Alternatively, it is also possible to provide a pressure-releasing lever by which the pressure can be released.
In the thermal printing device provided in the facsimile apparatus, even when printing is terminated, the platen roller serves to press the thermosensible paper against the thermal head. The force of pressure is as high as 4 Kg for 200 mm length of the thermosensible paper. This force is considered to be relatively high. The facsimile apparatus has to constantly keep the waiting state until the electric picture signals are received. Hence, the pressure is kept for a considerable time.
The platen roller is formed of an elastic material such as rubber so that the thermosensible paper becomes suitable to conveyance and printing. The platen roller pressed by the thermal head for a considerable time is transformed, resulting in causing an eternal compression distortion on the platen roller and implementing no exact printing.
To reduce such an adverse effect, it is necessary to use an expensive rubber material on which an eternal distortion is hardly caused. It results in enhancing the manufacturing cost of the facsimile apparatus. Further, since a larger pressure is caused for a considerable time, the thermal printing device has to be stiff enough to be transformed by the reaction of the platen roller. It is therefore difficult to reduce the thermal printing device in size.
In turn, a third disadvantage of the thermal printing device will be described.
When an abnormal state takes place in conveying the thermosensible paper, printing of the image data cannot be properly carried out in synchronous to the conveyance of the thermosensible paper as well as the thermosensible paper may be stuffed in the conveying direction so that an excessive burden may be applied on the mechanism for conveying the thermosensible paper, resulting in partially damaging the thermal printing device. The thermal printing device, therefore, senses an abnormal state, that is, a paper jam in the conveying direction so that the thermal printing device immediately stops to function if the abnormal state is sensed.
In this case, the thermal printing device stops its operation as it is, that is, in the state that the platen roller is pressed against the thermal head when an abnormal state is sensed in the paper conveying path. In order to remove an abnormal state in the conveying direction, therefore, it is necessary to remove the cause of the abnormal state appearing in the paper conveying path after the pressure caused between the platen roller and the thermal head is released.
In order to remove the abnormal state in the conveying path without releasing the pressure of the thermal head against the platen roller, a portion of the thermosensible paper is torn out as it is tightly held between the thermal head and the platen roller. Hence, it is more difficult to remove the torn portion.